The present invention relates to an image forming apparatus in a copying machine, printing machine and the like and more particularly, to an image forming apparatus which is arranged to correct a dislocation of the leading edge of a paper sheet to correspond with the position of an image.
In a conventional image forming apparatus, the timing for turning on register rollers which are provided for feeding a paper sheet and the timing for starting exposure are predetermined so as to always maintain a predetermined correlative relations between the edge of a paper sheet and the position of an image.
Referring now to FIG. 4, designated by numeral 1 is a photosensitive drum and to the predetermined position of the surface of which laser beam 2 is irradiated for exposure. On a paper feed path leading to transfer section which is disposed around the photosensitive drum, paper feed rollers 4, first paper sensor 5, register rollers 6, second paper sensor 7, transfer and separation chargers 8 are sequentially disposed in the order described. When the distance a-d is set with system speed X as shown in FIG. 4, the time T (time for movement shown by slant lines) required for the register rollers turning on after starting exposure comes out; EQU T=(a-c-d)/X..... (1)
as far as computation goes.
However, in a practical movement of a paper sheet, the paper can not be carried just as per the calculation made above because of various causes arising from inconstant start and stop of the register rollers, variable conditions when the paper dashes to enter into the register rollers. Besides, there is delicate differences in functioning of each machine with another difference arising from the period of time the machine used, thereby causing dislocation of the leading edge of the paper to the position of an image when transfer is made.
In order to avoid such mechanical disagreement, U.S. Pat. No. 4,059,833 discloses a device which is arranged to either turn on register rollers or start exposure by use of basic signals. A time measuring means is provided therein for measuring the time after the basic signal is emitted, and by the output from the time measuring means, exposure is started or register rollers are turned on, and the positions of a paper sheet and an image is corresponded by adjusting the set value of the time measuring means. However, such device known already require troublesome work for the adjustment once the positions of a paper and an image became not to correspond to each other.